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*''E. coli'' is part of the flora of the large intestine, but is not usually found in the small intestine
*''E. coli'' is part of the flora of the large intestine, but is not usually found in the small intestine
*Some strains possess fimbrae which attach the bacteria to the small intestinal epithelium of particular animal species
*Some strains possess fimbrae which attach the bacteria to the small intestinal epithelium of particular animal species
*''E. coli'' may cause diarrhoea via attaching and effacing lesions, where bacteria adhere intimately to the enterocyte, and cause localised effacement of the brush border microvilli; the epithelial erosion causes dysentery
*''E. coli'' may cause diarrhoea via attaching and effacing lesions, where bacteria adhere intimately to the enterocyte, and cause localised effacement of the brush border microvilli; the epithelial erosion causes [[General Pathology - Haemorrhage|Dysentery|dysentery]]
*Enterotoxigenic ''E. coli'' (ETEC):
*'''Enterotoxigenic ''E. coli'' (ETEC)''':
**General:
**General:
*** Contributes to [[Intestines - Catarrhal Enteritis#Undifferentiated Neonatal Calf Diarrhoea|undifferentiated neonatal calf diarrhoea]], a mixed viral enteritis in calves, also known as enteric colibacillosis
*** Contributes to [[Intestines - Catarrhal Enteritis#Undifferentiated Neonatal Calf Diarrhoea|undifferentiated neonatal calf diarrhoea]], a mixed viral enteritis in calves, also known as enteric colibacillosis
***K88 (F4) is associated with adhesion to the small intestinal mucosa of pigs
***K88 (F4) is associated with adhesion to the small intestinal mucosa of pigs
***K99 (F5) associated with adhesion in pigs and cattle (these fimbrial adhesins were originally thought to be capsular (K) antigens)
***K99 (F5) associated with adhesion in pigs and cattle (these fimbrial adhesins were originally thought to be capsular (K) antigens)
***The fimbriae are encoded by plasmids
***The '''fimbriae are encoded by plasmids'''
***These strains carry a plasmid which encodes an enterotoxin
***These strains carry a plasmid which encodes an enterotoxin
***Two types of enterotoxin: heat-labile (LT) and heat-stable (ST) toxins
***Two types of '''enterotoxin: heat-labile (LT) and heat-stable (ST) toxins'''
***The plasmids which produce these toxins are responsible for the pathogenicity of these strains
***The '''plasmids''' which produce these toxins are '''responsible for the pathogenicity''' of these strains
***LT is an oligometric toxin composed of an enzymatically-active A subunit (30KDa; 2 fragments - A1 and A2) and 5 identical B subunits (12KDa) forming the binding portion (B oligomer)
****'''LT'''
***It attaches to the brush border of the epithelial cells of the small intestine
*****An oligometric toxin composed of an enzymatically-active A subunit (30KDa; 2 fragments - A1 and A2) and 5 identical B subunits (12KDa) forming the binding portion (B oligomer)
***LT causes ADP-ribosylation of the stimulatory subunit of guanine nucleotide binding proteins of the adenylate cyclase complex in eukaryotic cell membranes
*****It attaches to the brush border of the epithelial cells of the small intestine
***This causes irreversible activation of adenylate cyclase in target cells
*****Causes ADP-ribosylation of the stimulatory subunit of guanine nucleotide binding proteins of the adenylate cyclase complex in eukaryotic cell membranes
***This raises the cAMP level and causes hypersecretion of water and chloride ions into the lumen of the small intestine and inhibits reabsorption of sodium
*****This causes irreversible activation of adenylate cyclase in target cells
***ST activates guanylate cyclase in enteric epithelial cells, stimulating fluid secretion
*****This raises the cAMP level and causes hypersecretion of water and chloride ions into the lumen of the small intestine and inhibits reabsorption of sodium
****'''ST'''
*****Activates guanylate cyclase in enteric epithelial cells, stimulating fluid secretion
**Clinical signs:
**Clinical signs:
***The gut becomes distended with fluid and a [[Intestines - diarrhoea#Secretory Diarrhoeas|secretory diarrhoea]] which lasts several days results
***The gut becomes distended with fluid and a [[Intestines - diarrhoea#Secretory Diarrhoeas|secretory diarrhoea]] which lasts several days results